Paint state transition for overlay 9 patch scrollbars

ui/native_theme/native_theme.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef UI_NATIVE_THEME_NATIVE_THEME_H_
 #define UI_NATIVE_THEME_NATIVE_THEME_H_
 
 #include "base/observer_list.h"
 #include "third_party/skia/include/core/SkColor.h"
 #include "ui/gfx/native_widget_types.h"
@@ skipping 205 matching lines @@
                                 State state,
                                 const ExtraParams& extra) const = 0;
 
   // Paint the part to the canvas.
   virtual void Paint(SkCanvas* canvas,
                      Part part,
                      State state,
                      const gfx::Rect& rect,
                      const ExtraParams& extra) const = 0;
 
+  // Paint part during state transition, used for overlay scrollbar state
+  // transition animation.
+  virtual void PaintStateTransition(SkCanvas* canvas,
+                                    Part part,
+                                    State startState,
+                                    State endState,
+                                    double progress,
+                                    const gfx::Rect& rect) const { }
+
   // Supports theme specific colors.
   void SetScrollbarColors(unsigned inactive_color,
                           unsigned active_color,
                           unsigned track_color);
 
   // Colors for GetSystemColor().
   enum ColorId {
     // Windows
     kColorId_WindowBackground,
     // Dialogs
@@ skipping 103 matching lines @@
  private:
   // Observers to notify when the native theme changes.
   ObserverList<NativeThemeObserver> native_theme_observers_;
 
   DISALLOW_COPY_AND_ASSIGN(NativeTheme);
 };
 
 }  // namespace ui
 
 #endif  // UI_NATIVE_THEME_NATIVE_THEME_H_